Window air-conditioner units are a reliable and simple-to-install solution to keep a room cool while avoiding the costly construction of a central air system. Better yet, when the summer heat dies down, these units can be easily removed for storage, and you can use the windowsill for other purposes.Window AC units come in various sizes and cooling capacities, and it is important to choose the one that best fits the needs of the room. Check the square footage of the room you need to cool and match that to a window AC unit. If the room is unusually hot or cool, you can adjust the air-conditioner cooler capacity accordingly.Other factors that affect the performance of an air-conditioner are number of people in the room and how large the doorways are into other spaces.